In the field of endodontics, one of the most important and delicate procedures is that of cleaning or extirpating a root canal to provide a properly dimensioned cavity while essentially maintaining the central axis of the canal. This step is important in order to enable complete filling of the canal without any voids and in a manner which prevents the entrapment of noxious tissue in the canal as the canal is being filled.
In cleaning and enlarging the root canal, instruments called files, or reamers, are used. Commonly, both files and reamers have flutes which spiral along a portion of the instrument length and define cutting edges used to shape the root canals. The classification of an instrument as a file or a reamer depends upon the pitch of the flutes. A instrument having a greater pitch enables the instrument to cut better in a rotary mode, hence the instrument is a reamer. An instrument having a lesser pitch enables the instrument to cut better in the reciprocating mode, hence a file. During a cleaning and shaping operation performed with such a file or reamer, the instrument is normally rotated and moved into and out of the root canal along the longitudinal axis of the instrument.
Root canal files are available in various cross-sectional geometries and typically have two, three, four or even more cutting edges. Files can be classified as either landed or non-landed files depending on the geometry of the cutting edge. Non-landed files generally have a sharp cutting edge which extends from a central axis of the file as depicted in FIG. 2 which illustrates a cross-section of a non-landed file 110 having three flutes 112 and three cutting edges 114, 114a and wherein the spiral configuration has been removed for clarity. The sharp cutting edge 114 is relatively weak and therefore may be distorted or folded back upon itself in a direction opposite the cutting direction CD if excessive forces are applied while in use, as illustrated by cutting edge 114a. Such distortion of the cutting edge 114a severely lessens its effectiveness or makes the instrument 110 unusable.
Landed files were developed to address the strength problems of non-landed files. Referring to FIG. 3, the cutting edges 122 of landed files 120 do not terminate in a sharp point. Instead, they are reinforced with a thick section of material directly behind the cutting edge 122, creating a lobe-like protrusion with the cutting edge 122 at the leading edge of a land surface 124. While landed files exhibit increased strength at the cutting edge, they also significantly increase the friction between the file and the tooth. This increased friction generates undesirable heat in the canal and increases the stress on the file, which can lead to failure and breakage of the instrument. Indeed, one of the problems with traditional endodontic instruments used for extirpating and filing root canals is that the torsional limitations of the instrument are often exceeded, resulting in breakage of the instrument.
In an attempt to address the increased friction of landed files of the type depicted in FIG. 3, files with reduced land surface areas have been developed, as depicted in FIG. 4. This modified-land file 130 has three flutes 132 defining three cutting edges 134 at the leading edge of a reduced surface area land 136. The remaining portion 138 of the land 136 is stepped down to reduce the amount of surface area in contact with the tooth. While these modified-land files provide improved strength, compared to non-landed files, they still generate undesired friction within the canal.
There is thus a need for an endodontic instrument which exhibits improved strength at the cutting edge and overcomes problems of prior art endodontic instruments, such as those described above.